Composite materials enjoy widespread acceptance, with increasing popularity, as structural materials in many engineering designs. Offering the designer a high strength-to-weight ratio, good stiffness and modulus characteristics, and typically an environmentally benign electrochemical activity, composites are now replacing many structural alloys in a broad spectrum of engineering applications. The transportation arts are notable in the integration of composite structures in several significant ways, including ground vehicles but with heavier emphasis on aerospace applications. For example, aircraft and missiles have now seen considerable incorporation of nonmetallic composite members through out these airborne vehicles.
Where the designer employs a composite as a structural element, the fixturing of same has posed some problems. If one intends to use a composite structure, for example, as a flooring or bulkhead member there are various fixturing techniques which may be employed. Usually, in those situations, one is concerned with a transverse compressive force across the composite structure, including any shape-defining core member, such that the loads imposed tend to avoid forces having substantial shearing components.
On the other hand, the full utility of composite members has yet to be realized where a structural or load-bearing element must be fixtured in such a way as to induce shear forces or bending moments as respects the fiber orientation of the composite. One environment, and indeed the most preferred for purposes of the present invention (where full integration of composite structures has historically been limited), is that involved in respect of airfoil components of an aircraft or missile. These would include, for example, an elevon, wing, fin, or like primary airfoil surface which is subjected to substantial force and which, within that environment, must be reliably manipulated to various positions or configurations to serve its intended purpose. Fixturing such a control surface for that purpose has proved to be problematic at best. The desirability of using composite structures with the advantages offered by high stiffness and strength-to-weight ratios, coupled with an ability to withstand the wide range of service temperatures and severe environments involved, has been at least partially offset by a difficulty in fixturing those airfoils.
One approach to the resolution of that problem within that specific environment has been the incorporation of doublers to improve structural integrity of the airfoil within the root region. For example, a current missile design includes an elevon having a composite graphite/epoxy skin bonded to a chopped fiber molding compound core to define the airfoil structure. During the fabrication of that elevon, a titanium doubler is disposed proximate each skin/core interface along with an appropriate molding agent. The elements are subjected to a high pressure pressing operation in order to unify the disparate members into a structurally integral component. Fixturing is then achieved by discrete fasteners disposed transversely through the root with the doublers serving for structural load transfer along the fixturing line. Quite clearly, that is a cumbersome design but one which is found to be an acceptable tradeoff where the designer wishes to merge the advantages of composite structure within such a craft. To date, that has been a limiting tradeoff not only respecting airfoil surfaces but the broader incorporation of structural composites within many other engineering endeavors.
With an appreciation for the problems associated with the transfer of shear and/or bending moments from a composite to a supporting structure, the patent art generally follows along the lines of adapting composites to those situations and indeed relegating them only to those situations where force transmission is by and large in a compressive nature. For example, U.S. Pat. No. 4,252,378 discloses a wheel laminate having a syntactic foam core, specifically designed to serve as an automotive wheel for a pneumatic tire. The approach disclosed there involves the use of two metallic rim and disc halves along with a syntactic foam core of a toughened thermoset material to provide a sandwich wheel structure for dampening noise and vibration transmitted from the road to the vehicle on which the wheel is employed. The use of integral rim and disc halves is said to provide wheels which may be more economically formed as, for example, by stamping. The foam core is disclosed to seal the seam weld while the same core material locates and retains the bolt hole spacer ring between the disc portions for the asserted purpose of contributing to the load carrying capability of the structure and economy of assembly. In a particularly preferred disclosed embodiment in that reference, the laminated structure is comprised of spaced shells of thin steel, rigid plastic or a combination thereof, wherein the space intermediate the shells is filled with a syntactic foam based on matrix materials such as polyester or epoxy foam. That foam is poured or injected into the shell cavity or may be preformed and adhesively sandwiched therein prior to curing. The reference discloses metal skins, as opposed to composite skins, and goes on to speak of the molded fiber reinforced plastic in a manner suggesting the use of chopped fiber molding compound or injection molded skins. The metal fitting is disposed in a central position with respect to the overall assembly and, consistent with normal wheel design, does not project from one end wherein bending moments or like shear forces would obtain, the latter a more critical design problem as respects the intended environment of the present invention. Other references within the patent art appear similarly restricted in use, among which may be mentioned U.S. Pat. No. 3,707,434 (dealing with a sandwich panel having a syntactic core, but without disclosure of a metal fitting); U.S. Pat. No. 3,968,996 (another wheel design incorporating soft polyurethane foam as a sandwich core, and again without a separate metal fitting); U.S. Pat. No. 3,996,654 (dealing with the molding of syntactic foam flotation models, without discussion of a metal load fitting); U.S. Pat. No. 4,000,926 (pertaining to a wheel with steel faces joined by foam, otherwise not specified as syntactic); U.S. Pat. Nos. 4,013,810 and 4,034,137 (generally describing fabrication of a sandwich structure); U.S. Pat. Nos. 4,035,028 and 4,153,657 (dealing with wheel designs which employ metal face elements and soft polyurethane foam cores); U.S. Pat. No. 4,250,136 (referring to methods for fabricating composite structures, but otherwise silent respecting ancillary metal fittings); U.S. Pat. No. 4,292,368 (pertaining generally to the use of foams to rigidize a flattened end of a tubular rod); and U.S. Pat. No. 4,401,715 (another disclosure within this ambit, but again failing to disclose or suggest separate metal fittings).
From the brief discussion of the art as aforesaid, one will quickly appreciate that composite structures including foam cores for shape definition and load distribution have seen very limited structural utility because of the pervasive problem of transferring shear and/or bending loads outwardly at one end of the device. While those designers involved, for example, in the aerospace arts would dearly love to incorporate composite structures more broadly throughout aircraft and/or missiles, the limitation on fixturing these articles in such a way as to avoid separation at the root had made this heretofore an elusive task.